There has been proposed a technique disclosing a terminal device which executes a plurality of communication applications (for example, IP FAX or email). The terminal device executes data communication using one of a plurality of packet networks based on different communication charges per unit packet. The terminal device uses a packet network with the lowest communication charges per unit packet when executing a specific communication application. According to the related-art technique, communication charges corresponding to the amount of data packets which were actually transmitted are imposed on a terminal device. This accounting system will now be called a “packet-based accounting system”.